The present invention relates to a thermal stable low elastic modulus material capable of relaxing a stress generated between bare chip and bare chip, bare chip and metal material, bare chip and organic material, bare chip and inorganic material, metal material and organic material, metal material and inorganic material, and organic material and inorganic material in semiconductor fields.
In electrically connecting a bare chip with a thin film multi-layered circuit formed on a printed circuit board, a thin film multi-layered circuit board, a ceramic circuit board or a metal board through a solder ball, that is, in the so-called flip chip packaging, there is left the following problem.
There is a difference in a coefficient of thermal expansion between the bare chip and the printed circuit board, thin film multi-layered circuit board, ceramic circuit board or metal board. For that reason, the stress concentrates on a solder ball and a fatigue fracture of the solder ball easily occurs at the time of a thermal cycle test during or after the production process.
As one of means for solving said problem, a space between the bare chip and the circuit board after completion of the flip chip packaging is filled with a resin referred to as under-fill, thereby increasing reliability of the electric connection between the bare chip and the circuit board. This is considered to be an effect of under-fill to distribute the stress concentrated on the solder ball.
As another means for solving the problem, JP-A-11-17346 discloses a process comprising using a printed board prepared by using an elastomer material, thereby relaxing the stress concentrated on the solder ball after completion of the flip chip packaging. Furthermore, JP-A-10-107172 also discloses a process comprising using an elastomer as an adhesive applied between the bare chip and the solder ball, thereby reducing the stress loading on the solder ball.
It is a trend that a packaging density becomes higher, a chip size increases much more and the solder ball becomes smaller along with the miniaturization of electronic instruments. In addition, the existing elastomer has a glass transition temperature (Tg), at which a low temperature region and a high temperature region are separated from each other, and therefore it becomes difficult to ensure reliability. The reason why it becomes difficult to ensure reliability is considered as follows. At the low temperature region below the Tg, elasticity of the elastomer material becomes high, thereby increasing the stress concentration on the solder ball, and as a result, the solder ball easily produces cracks. Whereas, above the Tg, deformation of the elastomer becomes large, thereby deteriorating an adhesion strength and an electric insulation.
As materials of little change in their physical properties above or below the Tg, JP-A-8-100107 and JP-A-2000-109709 disclose an epoxy resin material, and JP-A-7-331069 and JP-A-7-331070 disclose a polyamide-amide resin material. However, these materials are all high elasticity materials and therefore cannot be used as a low elastic modulus material for stress relaxation use.
In prior arts, those obtained by incorporating a rubber component in a thermosetting resin and those obtained by introducing a siloxane as a low elastic modulus component in a polymer have been used as a low elastic modulus material. However, these materials are those having a Tg within a range of from room temperature to 200xc2x0 C. In the case of a temperature higher than 200xc2x0 C. in comparison with the case of room temperature, decrease in the elastic modulus becomes not less than two figures, and increase of coefficient of linear expansion becomes several times, and as a result, adhesion strength and electric insulation greatly deteriorate. Further, those obtained by incorporating a rubber component in a thermosetting resin have a Tg within a range of from room temperature to xe2x88x9250xc2x0 C. owing to the rubber component. Thus, below the Tg, elasticity of the rubber component is increased with increase in the stress between the bare chip and the circuit board. As a result, it is a trend that a size of the bare chip is increased with increase of peel of the bare chip from the circuit board to cause deteroration in electric insulation between the bare chip and the circuit board after completion of the flip chip packaging.
An object of the present invention is to provide a thermal stable low elastic modulus material, which is possessed of thermal stability of not less than 300xc2x0 C., capable of relaxing the stress between bare chip and bare chip, bare chip and metal material, bare chip and organic material, bare chip and inorganic material, metal material and organic material, metal material and inorganic material, and organic material and inorganic material in semiconductor fields, and high in electric reliability.
In order to accomplish the above-mentioned object, it is important to diminish change in physical properties of a low elastic modulus material, which change occurs above and below a Tg within a range of from xe2x88x9250xc2x0 C. to 300xc2x0 C. That is, it is important that an elastic modulus measured at xe2x88x9250xc2x0 C. of a low elastic modulus material is from 2 GPa to 0.01 GPa and an elastic modulus thereof measured at 300xc2x0 C. is not less than one fiftieth time that measured at xe2x88x9250xc2x0 C., a coefficient of thermal expansion thereof measured in the vicinity of xe2x88x9250xc2x0 C. is not less than 0.5 time that measured in the vicinity of 300xc2x0 C., and difference between an electric insulation thereof measured at 25xc2x0 C. and that measured at 200xc2x0 C. is less than three figures.
It is considered that in order to diminish the change in physical properties of a resin owing to a temperature fluctuation, it is important to incorporate a substance at the level of molecule, which substance is little in change in physical properties owing to a temperature fluctuation. The present invention is concerned with a technique, according to which an SiO2 skeleton, which is stable from a viewpoint of dynamic characteristics, and which is little in change in physical properties owing to a temperature fluctuation, is uniformly produced at the level of molecule, and a functional group capable of forming a covalent bond with a resin is imparted at the end of the SiO2 skeleton, so that it can be bonded with a resin, and thereby change of the resin owing to a temperature fluctuation can be diminished.
The present invention is summarized to contain a resin or resin precursor and an organosilicon compound represented by the following general formula 1, 2 or 3, provided that the resin contains at least one resin selected from the group consisting of polyimides, polyamide-imides and polyamides, and an elastic modulus measured at xe2x88x9250xc2x0 C. of a cured product of the resin is from 2 GPa to 0.01 GPa, 
wherein R is a group capable of forming a covalent bond with the polyimide, polyamide-imide or polyamide, and R1 to R6 are independently of one another a silicon-containing group having 0 to 10 repeating units of SiRO3/2 provided that when a number of the repeating unit is 0, R1 to R6 are independently of one another H, CH3 or C2H5.
With respect to a heat-cured resin obtained from a varnish comprising the above-defined resin or resin precursor and the above-defined organosilicon compound, its thermal stability is high, a glass transition point almost disappears, and difference of change in physical properties including elastic modulus and coefficient of thermal expansion within a temperature range of from xe2x88x9250xc2x0 C. to 300xc2x0 C. and electric insulation is little. Therefore, even when a size of the bare chip is increased, almost no peel of the bare chip from the circuit board and almost no deterioration in electric insulation between the bare chip and the circuit board can be observed after completion of the flip chip packaging.
When the thermal stable resin composition is cured, an integral value of peaks of from xe2x88x9253 ppm to xe2x88x9272 ppm obtained according to a 29Si-NMR chemical shift is from 1 to 30 times that of peaks of from xe2x88x9240 ppm to xe2x88x9252 ppm obtained according thereto. From the result, it is demonstrated that the organosilicon compound has an Sixe2x80x94Oxe2x80x94Si bond to increase the molecular weight. The thermal stable low elastic modulus material comprises a mixture of a resin varnish and a liquid organosilicon compound, which organic compound is uniformly distributed in the thermal stable low elastic modulus material.
The polyimides, polyamide-imides and polyamides, whose cured product has an elastic modulus of from 2 GPa to 0.01 GPa, are not particularly limited and may be one known in the art.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.